Large commercial irrigation systems such as those used on golf courses or croplands use sprinklers, sensors or other components that are normally powered from 24 V AC power lines that can be several miles long and can serve many hundreds of components. Various systems have been proposed for powering and controlling the components of such a system with just two wires. For example, U.S. Pat. No. 3,521,130 to Davis et al., U.S. Pat. No. 3,723,827 to Griswold et al., and U.S. Pat. No. 4,241,375 to Ruggles (each of which is incorporated herein by reference) disclose systems in which sprinkler valves along a cable are turned on in sequence by momentarily interrupting the power or transmitting an advance signal from time to time.
A problem with this approach is that it does not allow the operator to freely turn on or off any selected sprinkler or set of sprinklers at different times. This problem is typically resolved by providing separate controllers in the field to operate groups of sprinklers in accordance with a program stored in them, or transmitted to them by radio or other means. Alternatively, it has been proposed, as for example in U.S. Pat. No. 3,578,245 to Brock (which is incorporated herein by reference), to operate individual sprinkler sets from a central location by superimposing a frequency-modulated signal or DC pulses onto the 24 V AC power line.
All of these approaches are expensive. For example, a system with hundreds of sprinklers requires miles of expensive, heavy wiring to accommodate the current drawn by a large number of valves that may be watering simultaneously. Also, heavy use of D.C. current may cause electrolysis issues with electrical components.
One alternative to these traditional irrigation systems are two-wire power and communications systems, such as the system shown in U.S. Pat. No. 7,358,626, the contents of which are incorporated by reference. In such systems, two wires supply both A.C. power and digital control communications from a controller to a plurality of decoders.
While these A.C. power and digital communication systems generally work well, they also have disadvantages. These communication systems rely on at least two, electrically insulated wires buried within the ground. If the insulating layer on these wires is compromised, current can leak from the damaged wire into the ground. The copper within the wire begins electrolytic corrosion between the copper and the ground. As the wire corrodes, the amount of current that can be passed through the wire decreases until the corrosion passes completely through the wire, breaking the electrical connection.
Replacing damaged wire typically involves digging up long portions of the wire and therefore can be expensive. Additionally, it can be difficult to determine exactly where a break has occurred.